Many medical diagnostic tests are performed in a medical laboratory, on serum and plasma. Serum is the yellow liquid obtained from whole blood after the blood is allowed to clot, and the clot is removed by centrifugation; plasma is the yellow liquid obtained from blood by centrifugation of blood mixed with an anticoagulant, e.g. heparin. Whole blood comprises the formed elements (i.e., the cellular components and the cell-derived components), and plasma. Red blood cells are the most abundant formed elements in blood, and platelets are examples of cell-derived components.
Measurement of the concentration of therapeutic drugs and certain hormones are essential in patient management, and usually, the total concentration of the drugs and hormones are measured because with the exception of a few hormones, e.g. thyroid hormones, the tests or assays are designed to measure the total concentrations. Designing an assay to measure the concentration of the free drugs/hormones are more complex. Only the free therapeutic drugs, ions and free hormones are available to cross vascular walls and biological membranes in order to produce biological activity, by attaching to specific and non-specific binding sites or receptors. Some examples of a therapeutic drug, an ion and a hormone are phenytoin, calcium and cortisol respectively.
Phenytoin, for example, is a therapeutic drug used to treat epilepsy. In the blood, about 90% of the phenytoin is bound to plasma proteins. Only the portion of phenytoin that is unbound or “free” is pharmacologically or biologically active. A test for total phenytoin represents the sum of the bound and unbound phenytoin. Under normal conditions, the balance between bound and unbound phenytoin in the blood is relatively stable, so measuring the total phenytoin is appropriate for monitoring therapeutic levels of phenytoin. However, in certain conditions and disease states, that balance can be upset, causing the percentage of free or active phenytoin to increase. Consequently, a patient may experience symptoms of phenytoin toxicity even though the total phenytoin concentration falls within a therapeutic range. In such cases, doctors may order serum or plasma free phenytoin in order to more reliably monitor the patient's phenytoin levels, instead of serum or plasma total phenytoin.
One method used to measure free phenytoin in a patient's serum or plasma sample involves: 1) adding the patient's sample to the sample reservoir of an ultra-filtration device; 2) capping the sample reservoir; 3) placing the ultra-filtration device in a centrifuge and centrifuging for about 25 minutes; and 4) measuring total phenytoin in the ultra-filtrate of the serum or plasma.
By way of examples only, some embodiments of a filtration apparatus that can be used to extract plasma from whole blood can be found in U.S. Pat. Nos. 7,816,124 and 7,807,450 awarded to the inventor. Subsequently, the inventor filed U.S. patent application Ser. No. 13/549,443 entitled “Sample Filtration Apparatus”, which describe other embodiments of filtration assemblies.
In the case of a serum or plasma sample, the filtrate (or more appropriately, referred to as an ultra-filtrate since plasma is already considered to be a filtrate of whole blood) usually refers to the serum or plasma containing the smaller molecular weight substances like the free phenytoin, and the retentate usually refers to serum or plasma containing the higher molecular weight substances like the proteins that bind phenytoin. An example of such a protein is albumin, having a molecular weight of about 66 kilodaltons. In contrast, the molecular weight of phenytoin is about 0.25 kilodaltons. A person of ordinary skill in the art will appreciate that an ultra-filtrate is still a filtrate, and the term ultra-filtrate is used for clarity when the filtrate contains substances having low molecular weights relative to the molecular weight of large dissolved substances, for example large proteins like immunoglobulins. Also, it seems appropriate to call the fraction of plasma having the smaller molecular weight substances a plasma ultra-filtrate, since the starting sample is plasma, which is already considered to be a filtrate of blood.
U.S. patent application Ser. No. 13/549,443 filed by the inventor describes cartridges for extracting plasma and serum ultra-filtrate, but the devices can only be operated manually. Moreover, some embodiments of these devices require at least one manually operable compression chamber. Moreover, in operation the sample ultra-filtration chamber is not vented to the atmosphere. There is a need for an ultra-filtration cartridge that can be used in an automated laboratory system, where centrifugation is not required and manual handling of samples are minimal.
Laboratory automation is a strategy to develop, optimize and capitalize on technologies in the laboratory that enable new and improved processes for reducing laboratory process times. The most widely known application of laboratory automation technology is laboratory robotics. More generally, the field of laboratory automation comprises many different automated laboratory analyzers, devices, software algorithms, and methodologies used to enable, expedite and increase the efficiency and effectiveness of providing test results.
The automated process of providing plasma and serum ultra-filtrates, for example, can be incorporated in laboratory automation, and the plasma and serum ultra-filtrates used to measure therapeutic drugs, ions and hormones, for example.